1. Field of the Invention
The present invention relates generally to energy transfer devices, and relates more particularly to power inverter circuits with reduced component rating.
2. Description of Related Art
Typical power systems for transferring energy between an input and an output often employ a power inverter that has a DC input and a switched output that can be single or multiple phase. For example, referring to FIG. 1, an inverter for driving a three phase motor M is illustrated generally as inverter 10. Inverter 10 operates by switching the power supplied by the plus and minus DC bus lines into motor lines U, V and W to operate motor M. Switches 12a–12f are switched on and off to appropriately direct power to and from motor M in dependency upon the desired power output, control scheme, available DC bus power and other parameters that factor into obtaining a high performance motor drive. Each pair of switches connected between a +DCBUS line and a −DCBUS line form a switching half bridge for delivering power to motor lines U, V and W. For example, switches 12a and 12b form a switching half bridge to drive power signals on motor line U.
Operation of the half bridge formed by switches 12a–12b is accomplished through standard switching practices to avoid problems associated with component limitations such as switching losses, and to improve system performance. Accordingly, switches 12a and 12b are never switched on at the same time to avoid current shoot through in the motor drive. In addition, a dead time is provided between switching intervals when both switches in the half bridge change state. For example, if switch 12a is to be turned off and switch 12b is to be turned on, these events do not occur simultaneously, but with a delay between switch 12a turning off and switch 12b turning on. When a high frequency inverter drive is used for high performance motor control, the dead time delay becomes important to improve switching frequency without incurring the above discussed drawbacks.
High frequency switching also produces rapid changes in power transferred from the inverter to the motor and vice versa. These rapid changes in transferred power implies the need of higher power ratings for the switches in the inverter, for example, to handle the potentially large range of power fluctuations.
Similarly, other components coupled to the inverter, such as passive energy storage components, are rated to withstand potentially large power fluctuations including high peak currents and voltages and large ripple currents and voltages. Referring to FIGS. 2A and 2B, a passive electrical energy storage component is coupled to inverter 10 to both store input energy, and supply stored energy through inverter 10. FIG. 2A shows storage capacitor CBUS coupled between the DC bus lines input to power inverter 10, while FIG. 2B shows inductor Li in the positive DC bus line connected to power inverter 10. In FIG. 2A, capacitor CBUS stores electrical energy for power inverter 10 acting as a voltage source inverter, while inductor Li in FIG. 2B acts as a DC link inductor for power inverter 10 acting as a current source inverter. Due to the difficulties associated with energy transfer in power inverter 10 discussed above, the power ratings for the storage components CBUS and Li must be selected to be high enough to handle the fluctuations in power without saturating or damaging the passive energy storage components.
When selecting appropriately rated passive components for use with power inverter 10, the components with appropriate ratings are typically large and somewhat expensive. For example, a typical bus capacitor CBUS comprises a large percentage of a motor drive size and cost. It would be desirable to reduce the rating, and thus the size and cost, of the passive components used with a typical motor drive system.